This invention relates to a novel method of forming siloxane bonds by mixing carboxylic acids with alkoxysilanes in the presence of catalytic amounts of strong acids.
It is well known in the art that when alkoxysilanes are hydrolyzed they first form silanols which can then condense to form siloxanes (see, eg., Noll: Chemistry and Technology of Silicones, Second Edition, Academic Press. NY (1968)). As described in this reference, one of the more common methods of hydrolyzing alkoxysilanes involves merely mixing the alkoxysilanes with water.
Noll suora, Sowa in U.S. Pat. No. 2,486,992 and Leznov et al. in J. Gen. Chem USSR. Vol 29, p. 1482 (1959) also describe conversion of organoalkoxysilanes into silanols by reaction with organic acids and a hydrogen ion source. The resultant silanols are thought to later condense to form siloxanes These references teach that 1 equivalent of organic acid is required for each equivalent of alkoxy group.
Langkammerer in U.S. Pat. No. 2,490,691 teaches the polymerization of tetraalkylsilicates to form alkylpolysilicates. The process described therein comprises mixing the tetraalkylsilicates with carboxylic acids at elevated temperatures. This process provides high molecular weight polymeric esters of polysilicic acid which are liquid and soluble in organic solvents.
Bayer in GB Patent 722,441 describes a process of producing siloxane bonds by hydrolyzing halosilanes. The process described therein comprises reacting an organohalosilane with a carboxylic acid and a compound containing an alcoholic or phenolic hydroxyl group.
Various method of hydrolyzing trichlorosilanes are likewise described in the art. For instance, U.S. Pat. Nos. 3,615,272 and 5,010,159 granted to Frye et al. and Bank et al. respectively describe processes for forming hydrogen silsesquioxane polymers in which trichlorosilane is hydrolyzed in a medium comprising sulfuric acid and an aromatic hydrocarbon. The resultant product is isolated by either washing the reaction mixture with water and sulfuric acid or by neutralization with a mild base.
Therefore, what has not been described in the prior art and has now been discovered is that an extremely wide variety of alkoxysilanes can be converted to siloxanes using 1/2 equivalent of carboxylic acid for each siloxane bond formed.